In recent years, cloud services, such as the hosting service and IaaS (Infrastructure as a Service), have become popular for the purpose of provisioning of virtual resources to a user. By making a contract with a service provider; a user can utilize, on demand, virtual devices such as virtual servers, virtual networks, virtual routers, virtual storages, and virtual load balancers (for example, Amazon Web Services (registered trademark), <http://aws.amazon.com/ec2>). For example, the user operates middleware such as an OS (Operating System), a DB (Data Base) server, a Web server, and a mail server on a virtual server; and can build his or her dedicated server without having to arrange for hardware.
Moreover, open source software for implementing the IaaS has also become popular and, for example, a large number of commercial cloud services based on OpenStack (registered trademark, the same applies hereafter) have been developed (for example, Rackspace (registered trademark) Public Cloud Powered by OpenStack, http://www.rackspace.com/cloud/).
Typically, virtual servers are primarily virtual machines operating in a hypervisor such as Xen (registered trademark, the same applies hereafter) or KVM (Kernel based Virtual Machine). However, a large virtualization overhead of hypervisors has led to the emergence of providers that provide container-type virtual servers having a smaller virtualization overhead or bare metal servers not accompanying virtualization. Thus, because of an increase in the types of servers such as bare metal servers, container-type virtual servers, and virtual machines; it is believed that the utilization of cloud services such as the IaaS will get promoted.